Due to either lack of self-ignition or unstable ignition, some kind of ignition assist source is required when employing a gas fuel, such as natural gas with low cetane value, as the fuel of a device in which the fuel is injected from a fuel injection valve into the cylinder of an internal combustion engine (engine) at the point when a piston is in the vicinity of the compression Top Dead Center and combusted in the same way as occurs in a diesel cycle. Accordingly, a continuously heated heater (glow plug) is provided as an ignition assist source in the cylinder head, and the fuel is injected through the fuel injection valve toward the heater.
The fuel injection, ignition and combustion processes employed in this system proceed as outlined below. First, during the intake stroke of the engine, air (EGR gas and air for an engine with an EGR device) is suctioned into the cylinder, this air and the like being compressed during a compression stroke to a high-temperature high-pressure state (approximately 500 to 600° C. and 50 to 70 Bar). At a position of the piston in the vicinity of the compression Top Dead Center, fuel (the gas fuel) is injected through the fuel injection valve as a spray toward the heater.
Thereupon, while a self-ignition will occur if the injected fuel is one of high cetane value such as petroleum, a self-ignition does not occur with a gas fuel such as natural gas which has a low cetane value, because of its high self-ignition temperature, which is over 900° C. Accordingly, in this case, a heater constantly maintained at high temperature is provided in a cylinder head as an ignition assist source, and the fuel is injected toward a glowing red-hot heater and ignited and combusted as a result of coming into contact with the heater.
However, an inherent problem of this device is the thermal fatigue of the heater that occurs due to the fuel striking the heater directly. That is to say, the fuel injected at high pressure through the fuel injection valve collides with the heater at comparatively low temperature as a result of adiabatic expansion occurring within the cylinder and causes a rapid cooling of the heater. At the same time, when the heater is exposed to the cylinder, the heater is exposed to the gas in the cylinder and heated and cooled repeatedly as a result of the changes in temperature of the gas, between approximately 2300° C. and air temperature (20° C.), that occur accompanying each of the intake, compression, explosion (expansion) and exhaust strokes. Accordingly, the heater suffers thermal fatigue and its duration of life is shortened. In addition, an ignition failure is also thought to occur as a result of rapid cooling of the heater caused by the fuel injected through the fuel injection valve colliding directly with the heater.
Accordingly, the heater employed in this type of system is normally covered with a shield. In a description of this system with reference to FIGS. 4 and 5, a heater (glow plug) G/P is fitted in a cylinder head C/H of an engine with the lower end thereof projecting from the lower surface of the head C/H, and a shield S is fitted to cover this heater G/P. In addition, a fuel injection valve IN is fitted in the approximate center position of a cylinder bore of the cylinder head C/H, and a plurality of injection holes IN/H are formed in the injection valve IN. The symbol P in the drawing denotes a piston, a combustion chamber C/C being provided as a depression in the top surface of the piston P. In addition, the symbol V denotes an intake valve or an exhaust valve.
The shield S covers the heater G/P at a prescribed interval from the surface of the heater G/P, has a cylindrically-formed side part SS and an essentially semicircular base part SB, and describes an overall bag-like shape (closed type). Holes S/H affording connection between the inside and the outside of the shield S are formed in the side part SS. Because the provision of the shield S prevents the fuel injected through the injection holes IN/H of the fuel injection valve IN from striking the heater G/P directly and, in addition, the heater G/P from being exposed directly to the combustion gas of the combustion chamber C/C, the thermal fatigue of the heater G/P is reduced and, as a result, its duration of life is extended.
The process from fuel injection to ignition and combustion with a heater G/P of a type covered by a shield S will be summarily described. Some (approximately 1/10 of the total) of the fuel (the gas fuel) injected at high pressure (100 to 250 Bar) through the injection holes IN/H of the fuel injection valve IN is injected toward the shield S while the remainder is injected toward and dispersed approximately uniformly into the combustion chamber C/C, each being continuously injected for a prescribed period. That is to say, one of the plurality of injection holes IN/H provided in the fuel injection valve IN is set in the direction of the shield S, the remainder being set in the circumferential direction of the inner wall of the combustion chamber C/C separated by an approximately equal interval.
The fuel injected toward the shield S decelerates as a result of collision with the side part SS thereof; after being warmed by the shield S, flows by way of the holes S/H into the shield S where the fuel is instantly heated to a high temperature by the heater G/P which has been pre-heated to a high temperature; and ignites when reaching its ignition temperature. Subsequently, the combustion gas within the shield S subjected to rapid volume expansion as a result of ignition is instantaneously jetted through the holes S/H. These holes S/H are orientated in the direction toward the vicinity of the remaining injection holes IN/H of the fuel injection valve IN (injection holes IN/H set in a direction that ensures spray and approximately uniform dispersal of the fuel sprayed into the fuel combustion chamber C/C).
For this reason, a flame that is spouted through the holes S/H of the shield S approaches the vicinity of the fuel spray injected through the remaining injection holes IN/H. That is to say, while a further increase in pressure and in temperature occurs across the fuel combustion chamber C/C as a whole as a result of ignition occurring within the shield S, the flame spouted through the holes S/H approaches the fuel spray injected through the injection holes IN/H because, at this stage, the fuel continues to be injected through the injection holes IN/H. As a consequence, ignition begins from the outer circumferential part of the spray consisting of an appropriate mix of air and fuel, and then combustion extends instantly to the remainder of the spray. The injection of the fuel is continued for a prescribed period thereafter and then finishes, whereupon the combustion process gradually finishes. The piston P is in its downward stroke at this time.
This fuel injection and ignition assist device for a cylinder injection-type internal combustion engine, which was developed and researched by the inventors of the present invention and their associates, did not constitute a publicly known art at the time of filing the application. However, examples of prior art references that disclose a heater G/P covered with a shield S include Japanese Utility Model Registration No. 2562423 and Japanese Examined Utility Model Publication No. H7-48982.
The stable operation of the engine requires the ignition and combustion processes described above to be repeated with precision of the order of p sec and, in addition, adequate stability even when the operating conditions (such as the fuel flow rate, intake-air temperature, pressure, intake-air flow rate, swirl strength, temperature of the overall engine) change.
The single most important key process is the contact of the fuel with the heater G/P that serves as the ignition source. In other words, the most important aspect of the process is how the fuel injected toward the shield S for ignition passes through the holes S/H of the shield S and stably (time-wise and volume-wise) contacts the glowing red-hot heater G/P standing by.
As shown in FIG. 6, the inventors of the present invention and their associates have conceived a device in which four holes S/H are provided in a side part SS of a shield S, the four holes S/H being arranged in a square shape on the side part SS, and an axis L1 (See FIGS. 4 and 5) of one injection hole IN/H for injecting fuel toward the shield S among a plurality of injection holes IN/H provided in the fuel injection valve IN being aligned with a perpendicular line L2 (line extending from a middle point X1 in the direction perpendicular to the paper of FIG. 6) extending perpendicularly from a middle point X1 of the four holes S/H of the side part SS.
Using this device type, as the fuel injected through the injection hole IN/H collides perpendicularly with the side part SS of the shield S at the collision point Y1 which is the fuel collision point on the side part SS (the same position as the middle point X1), and as the distances from the collision point Y1 to the holes S/H are the same, it was anticipated that, subsequent to collision, the fuel flows uniformly along the surface of the side part SS toward the four holes S/H and through the holes S/H into the shield S.
However, when the fuel flows along the surface of the side part SS and reaches the holes S/H under identical conditions in this manner, an approximately uniform pressure is produced in the holes S/H as a result of the action of the fuel. As a result, a state as if the holes S/H were covered by a virtual lid composed of the fuel was produced, and it became clear, in actual practice, that the fuel did not flow as easily as anticipated through the holes S/H into the shield S.
More particularly, in the closed type shield S of the illustrated example, the virtual lid composed of the fuel that closes the holes S/H is pressed against the rear side of the holes S/H at an approximately uniform force by the pressure noted above and, because of the absence of holes, passages or the like (see FIG. 4) through which the pressure within the shield S can be released at this time, the internal pressure of the shield S increases with the result that it becomes much harder for the fuel to flow through the holes S/H into the shield S. The present inventors and their associates confirmed this by testing and simulating.